Streptococcus pneumoniae strains are sensitive to the chemical optochin (ethylhydrocupreine hydrochloride).

Optochin sensitivity allows for the presumptive identification of alpha-hemolytic streptococci as S. pneumoniae, although some pneumococcal strains are optochin-resistant.

Other alpha-hemolytic streptococcal species are optochin-resistant.

Optochin
The growth of bacteria that are optochin-sensitive will show a zone of inhibition around a optochin disc, while the growth of bacteria that are optochin-resistant will not be affected.

Performing the optochin test
1) Using a inoculation loop, steak two or three suspect colonies of a pure culture to be tested on 5% sheep blood agar plate
2) Place an optochin disc (6mm, 5 µg) within the streked area of the plate
3) Incubate the blood agar plate at 35°C with 5% CO2 for 18-24 hours (culture do not grow well in ambient air, and larger zone of inhibations accur)

Reading the optochin test result
1) Positive: Zone of inhibiton is 14 mm or greater in diameter with 6 mm disk.
2) Negative: No zone of inhibition
3) Equivocal: Any zone of inhibition less than 14 mm is questionable for pneumococci, the strain is identified as a pneumocoocus only if its bile soluble.

Note
A smaller zone of inhibition (<14 mm) or no zone of inhibition indicates that the bile solubility test is required.
It is important to remember that pneumococci are sometimes optochin resistant.
 

Optochin(or ethylhydrocuprine hydrochloride) is a derivative of hydroquinine introduced in 1911 by Morgenroth and Levy with the intention to treat pneumococci infection.

In very high dilutions inhibitrs the growth of pneumococci in vitro.

That is the main reason it is now used in cell culture techniques for the presumptive identification of Streptococcus pneumoniae, which is optochin sensitive, from other alpha-hemolytic streptococci such as viridans streptococci, which are resistant.

Microbe Online
Optochin Sensitivity Test: Principle, Procedure, expected results and quality control.